This invention concerns a system for data transmission over fiber optics, the individual subscriber stations being connected by way of T-couplers spaced apart along a main fiber optics lead and the coupling-in and coupling-out of the light being effected directionally, so that two T-couplers are necessary for the connection of each subscriber station, which T-couplers form a structural unit designated a TT coupler, the incoming light being divided in the first junction of the TT coupler into two portions of which one passes directly to the receiver of the subscriber station and the second passes directly through the TT-coupler.
Such an arrangement of a TT-coupler with separate directional couplings has been proposed in German patent application No. P 2614051.2 dated Apr. 1, 1976.
Having regard to the optical and mechanical characteristics of the optical fibers, the extracting of light from a bundle of optical fibers or the injection of light into the bundle is for the time being possible only if, at the position concerned, the bundle is completely interrupted and appropriate coupling elements are interposed between the several ends. For the receiver itself a partial interruption would also be possible, but not for the transmitter, since all the fibers of the outgoing bundle must always be energized by the new signal from a subscriber station.
Fiber optics T-couplers of different constructions are known. For example, in the T-coupler shown in U.S. Pat. No. 3,883,217 two series-arranged solid glass cores are interposed between the several faces of the interrupted fiber bundle and the leads to the receiver and from the transmitter and also a direct connection are applied to their internal surfaces. The incoming light signal is thus divided into a portion going to the receiver and a portion passing directly through the coupler. The light passing directly through the coupler and the transmitter signal introduced in the same direction are combined at the output of the TT-coupler into a new transmitted signal. The two solid glass cores arranged in series have the function of a diffuser or scrambler. Light introduced at a point ultimately distributes itself, after traversing a certain length of core, uniformly over the whole core cross-section and thus passes into all the fibers of the outgoing bundle.
In accordance with U.S. Pat. No. 3,870,396 there are interjected between the point faces of the main lead, means including right-angle isoceles triangular glass prisms with their hypotenuse surfaces bridged by a mirror inclined relative to the direction of transmission of the light. This mirror is partly reflective at the position at which energy is to be coupled out to the receiver and fully reflective at another position for the introduction of the transmitter signal. The arrangement operates as a directional coupler, so that two such couplers are required to provide go and return paths.
An objection of principle to the TT-coupler is the relatively high insertion loss of 3-4 dB, resulting from the loss to the ballast and sheath cross-section of the outgoing fiber bundle. Various investigations have shown that it is extremely difficult to reduce this attenuation to a perceptible extent by measures taken at the coupler itself. Referred to a maximum allowable transmission attenuation of, for example, 40 dB between two stations, the main lead may, apart from the fiber attenuation, contain at most 10 such TT-couplers if operation without intermediate amplifiers is desired. Applications are of course conceivable in which the series loss of the couplers is of less importance.
In modern power installations up to 500 connections must be taken into account for data interchange. Intermediate amplifiers must be used here in any case, even if it were possible to reduce the passage attenuation of the TT-coupler by a factor of 10.
It has also been proposed to connect the subscriber stations directly in the main lead, through the employment of intermediate amplifiers. This has the disadvantage that if one station fails, all data interchange is interrupted. It would in fact be possible in principle to provide a direct optical bridge across a defective station, for example by using a Kerr cell or Pockels cell, which procedure is however quite expensive and in no case is without optical problems.
It is a further disadvantage of the known coupler systems that in some cases a certain amount of the transmitted signal may pass directly into the receiver branch of the station's own receiver. In certain applications, such as for example the arrangement according to the invention described below, such a condition is not possible.
Here only a TT-coupler with practically complete decoupling between transmitter and receiver paths can be taken into consideration, such as is described for example in German patent application No. P 2614051.2 already mentioned hereinabove.